Effect of partial substitution of 3d transition metals for Co on the thermoelectric properties of NaxCo2O4

Layered P2 type NaxCo2O4 thermoelectric oxides partially substituted by 3d transition metal (up to 15 at%: Cr, Mn. Fe, Ni, or Zn) for Co were synthesized through calcination and sintering in air. The Cr-substituted samples were mostly decomposed into the NaCrO2 + Co2CrO4 phases, On the other hand, the X-ray diffraction patterns of the Mn-substituted samples showed a single P2 type Y-NaxCo2O4 phase. The Fe- and Ni-substituted samples were mostly composed of the NaxCo2O4 phase with small amounts of Co3O4 and CoNiO2 phases. In the case of the Zn-substituted samples, only the Na-x(Co-0.95)(2)O-4 as mostly composed of the NaxCo2O4 phase with a small amount of the NaO phase. The effects of partial substitution of these 3d transition metals for Co except for Cr in the layered NaxCo2O4 on thermoelectric properties were investigated. The electrical resistivity of these partially substituted samples significantly increased over the entire temperature range, as compared to the non-substituted sample, indicating reduction in carrier concentration. Except for Na-x(Co1-yFey)(2)O-4, the Seebeck coefficient was enhanced by the partial substitution, and in the case of Na-x(Co1-yFey)(2)O-4, it is suggested that the decrease in the Seebeck coefficient is associated with reduction in the electron-electron correlation due to the substitution. For the Mn-substituted samples, a small grain size and many pores compared to the non-substituted sample A ere effective for reducing the thermal conductivity. The figure-of-merit Z was enhanced in the case of the Na-x(Co0.95Zn0.05)(2)O-4 above 550 K and the maximum dimensionless figure-of-merit ZT, 0.68, was obtained at 965 K.